Pandas get column value based on row value

I have the following dataframe:

df = pd.DataFrame(data={'flag': ['col3', 'col2', 'col2'],
                        'col1': [1, 3, 2], 
                        'col2': [5, 2, 4], 
                        'col3': [6, 3, 6], 
                        'col4': [0, 4, 4]},
                        index=pd.Series(['A', 'B', 'C'], name='index'))

For each row, I want to get the value when column name is equal to the flag.

– Index A has a flag of col3. So col_val should be 6 because df['col3'] for that row is 6.
– Index B has a flag of col2. So col_val should be 2 because df['col2'] for that row is 2.
– Index C has a flag of col2. So col_val should be 4 because df['col2'] for that row is 3.

>Solution :

Per this page:

idx, cols = pd.factorize(df['flag'])
df['COl_VAL'] = df.reindex(cols, axis=1).to_numpy()[np.arange(len(df)), idx]

Output:

>>> df
       flag  col1  col2  col3  col4  COl_VAL
index                                       
A      col3     1     5     6     0        6
B      col2     3     2     3     4        2
C      col2     2     4     6     4        4